The cyclospora parasite causes intestinal illness as a result of consuming contaminated food or water. Symptoms can begin a week or more after consuming the parasite. They include diarrhea and frequent, sometimes explosive bowel movements, according to the US Centers for Disease Control and Prevention. Those who are infected might also experience loss of appetite, weight loss, stomach cramps or pain, nausea, gas and fatigue. Vomiting, headache, fever, body aches and flu-like symptoms can also occur.

The illness can last from a few days to a few months and patients might feel better, then get worse again. Patients can be treated with antibiotics.

According to the Illinois Department of Public Health, the illnesses have been reported across multiple counties in the state, although no counties were named.

“The initial investigation indicates a link to consumption of McDonald’s salads produced for McDonald’s restaurants. Approximately one-fourth of Illinois cases reported eating salads from McDonald’s in the days before they became ill,” the health department said in a statement. The agency is asking that anyone who has eaten a McDonald’s salad since mid-May and experienced diarrhea and fatigue to contact a health care provider to be tested and receive treatment.

Iowa health officials are asking the same of anyone who has developed symptoms after consuming one of these salads since the middle of June. “This summer there have been several clusters of Cyclospora illness associated with various foods that are commercially available,” a statement from the Iowa Department of Public Health said.

“We’re working with CDC, FDA, and Iowa to investigate multiple potential sources,” Illinois health department spokeswoman Melaney Arnold said in an email.

McDonald’s is cooperating with the CDC and US Food and Drug Administration in investigating the outbreak, according to the state health departments.

McDonald’s said in an email it has been in contact with public health authorities from both states about an increase in Cyclospora infections. “Out of an abundance of caution, we decided to voluntarily stop selling salads at impacted restaurants until we can switch to another lettuce blend supplier,” the email said. “We are in the process of removing existing salad blend from identified restaurants and distribution centers — which includes approximately 3,000 of our U.S. restaurants primarily located in the Midwest.”

The parasite cyclospora is the culprit behind an ongoing outbreak linked to recalled Del Monte Fresh vegetable trays; it’s linked to more than 200 illnesses in four states.

The parasite is also to blame for an outbreak in Texas, although officials have not yet identified what’s making people sick there.